Heat Illness and Mountain Biking

This comes from the Paramedic side of me. Riding today in the heat made me think of the extreme potential we have here in the foothills for exercise induced heat illness and injury. So I thought I might share some thoughts on the matter, and hopefully get other people to comment on what they do, and what they know, regarding mitigating overheating.

There are four ways that the body will lose heat:

Conduction

Convection

Radiation

Evaporation

Convection and Evaporation seem to go together in my mind, and are the most effective way to cool the body. The preferred treatment for an overheating body that has not yet fallen victim to Heat Stroke is to “splash water on the patient, coupled with air fanning” (Auerbach). Water will help transfer heat away from the body up to 25x faster than if it weren’t there, so it’s a great tool for rapid cooling. However, it may take a fair amount of water. Dr. Auerbach states that it takes 100 mL of evaporated water to cool the patient 1 degree C (1.8 F). So if you’ve got someone who’s rippin’ hot, you can expect it to take a fair amount of water to get them cooled off. Also, when the temperature outside gets higher than our body’s temperature (~98.6) evaporative cooling is really the only way to drop heat.

Knowing that you are overheating is a difficult thing to assess when coupled with exercise. It’s normal for our bodies to heat up as we burn calories and spend time in the sun. The question is how much is too much, and am I reaching a point where I need to start intervening. Checking someone’s skin, oral, axillary, and rectal temperatures are helpful, but not necessarily indicative of your actual core temperature and how your body is handling it. All of these sites for checking your temp will be different based on the local blood flow, proximity to the outer shell, local metabolic rate, etc. So the best indicator is the person. If you think you’re overheating, go with that, stop and work on cooling down.

That being said, the body is an amazing little engine. It is very adaptive and with the proper training can be extremely efficient at handling high core temperatures. “One marathon runner was able to maintain a core temperature greater than 41.5 C (106.7 F) for at least 44 minutes during a race” and “one heatstroke victim survived a measured core temperature of 46.5 C (115.7 F)” (Auerbach). That’s pretty impressive, but this ability to survive excessive core temperatures for extended periods does not come without some serious acclimatization and training.

Signs and Symptoms of Heat Illness

There are three simple categories of heat illness:

Heat Cramps

Heat Exhaustion

Heat Stroke

Heat Cramps are brief, intermittent, potentially severe muscular cramps that are usually triggered by Sodium and water loss (and chloride). During periods of high heat we can lose 1-3L of water/hour through sweat, with each liter containing 10-60 mEq of sodium (230-1,380 mg of Sodium — One small bottle of Gatorade only has 230 mg of Sodium, or 23 mEq). Management is generally simple: remove from the hot environment, stop the activity, and try to replace sodium (electrolyte drinks, etc.). Occasionally, IV fluids may need to be administered, but this shouldn’t be the case unless the patient cannot hold oral fluids (e.g. they vomit). Because salt loss can reduce a person’s thirst signals, you may need to convince the person that they need to take on fluids. According to Auerbach, dietary intake of 170 mEq of sodium (3.9 grams), should be adequate to replace a conditioned athlete’s sodium loss during exercise. It’s interesting to note, as well, that it appears that with conditioning the sodium content in sweat may decrease…more acclimatization.

Heat Exhaustion is more severe and is generally accompanied by changes in mental status (irritability, poor judgment, etc.), dizziness, nausea, vomiting, headache, fatigue, heat cramps, profuse sweating, and mild to moderate elevation of core body temperature (up to 104 F, or 40 C). In severe cases, 2nd to volume loss, fainting may occur. Heat exhaustion, as its name suggests, is the result of the increased cardiovascular strain from the body trying to maintain normothermic levels, generally over a long period of time. Some of the symptoms may mimic those of a significant migraine. The biggest issues in heat exhaustion are still the ones we can fix: water and salt loss, along with other electrolytes, and associated volume loss (low blood pressure). The biggest concern is that if heat exhaustion is not treated, it can progress to heat stroke. So, take the victim out of the heat, begin cooling efforts (water and fanning), replace water, hopefully with electrolytes (sports drinks, electrolyte replacement tabs, etc.). If you don’t have any electrolyte replacements, use the water, but be careful not to “wash the patient clean” of their electrolytes (probably won’t be an issue). Too much water can cause hyponatremia (sodium loss), which can make the situation worse. If you find yourself fighting a delicate balance between loss of salt and loss of water (volume), which is a very difficult thing to assess, you need to get the patient to medical assistance.

Signs of excessive sodium loss are usually pretty easy to spot. The patient will have been drinking nothing but water (probably lot’s of it), sweating excessively, and will have a salty remnant on their skin and clothes, like in the picture below, but in greater quantities (this is my helmet and pack after a long ride in the hot sun, drinking only water…I was fine):

Heat Stroke represents a complete collapse of the patient’s thermoregulatory mechanisms. Their body temperature will rise to 105.8 F (41 C) or higher, the respiratory rate may quicken (to increase heat loss through exhaled air), mental status may range from a frank coma to confusion and irrational behavior (the brain is very susceptible to hyperthermia, so mental status changes will always be seen near time of onset, e.g. irritability, aggressiveness, stupor, delirium, etc.), and seizures or convulsions may be present. Pupils may also be constricted. About 25% of victims with exertional heat stroke will still be sweating and may sweat profusely, but in classic heat stroke, sweating is usually absent. The patient’s skin may be flushed, or exhibit a mottled appearance (which is worse than if it’s flushed), and the pulse rate will likely be high (a low pulse is an ominous sign and a signal for the need of immediate medical intervention as it may indicate cardiac failure). Patient may also complain of extreme nausea (this can also often occur with heat exhaustion), vomiting or diarrhea. Treatment for this patient needs to occur quickly. Definitive treatment, though controversial, is ice-bath or cold water immersion with vigorous massage. An appropriate substitute is saturation with water and aggressive fanning and massage to promote evaporative heat loss and circulation of cool blood. Keep a focus on the patient’s face and torso when doing this. Restrictive clothing should be removed, and obviously the patient needs to be removed from the heat source (when possible) and protected from further exertion (for example, the confused and irrational patient). A patient with true heat stroke needs medical attention because of the risk of organ damage. They need to be seen by a physician. Likewise, a patient who has significantly altered mental status needs to be evacuated to medical attention immediately. “Early warning signs [of heatstroke] include flushed face, hyperventilation, headache, dizziness, nausea, tingling arms, piloerection, chilliness, incoordination, and confusion” (Auerbach). (Piloerection is goose-bumps)

Thoughts

Something I didn’t know much about until reading it in Dr. Auerbach’s Wilderness Medicine book is something called Arterio-Venous Anastamoses (AVAs) which are found in the non-hairy, non-insulated portions of our bodies (i.e. palms of hands, soles of feet, and face). These AVAs are the primary locations for heat exchange/loss, and thus are a good area to focus on if someone’s in need of cooling. In the field, we are taught to focus on the body’s core…placing cold packs around the trunk of the body, rather than peripherally. Some of this is because we don’t want to send cold blood back to the heart, but new techniques that focus on these AVAs sound pretty cool. For instance, there is a company that invented a machine that a patient sticks their hand into and grabs a metal bar that is cooled to 72 F (if I remember right). Their hand is then sealed in this little machine via a vacuum in order to keep these AVAs from constricting, and the resultant cooling effects (and performance/recovery gains in athletes) is remarkable. Given that, lately, if I feel like I’m getting too hot, or just want to cool down, I’ll stick my hands in a stream or take them off the handlebars while riding to keep them cool. Also, I haven’t been wearing my fingerless gloves, except on long downhills. Whether psychosematic or not, I’ve found this to be very effective. I haven’t had many true heat emergencies in the field since learning about this, but I’m curious what the effects would be of having a patient massage an insulated cold pack with their hands. Massaging might keep the AVAs open, and with the packs being insulated, so as not to be too cold, we shouldn’t have to worry about shocking the heart with cold blood. Food for thought.

More Thoughts

A key point when cooling a patient is to be sure to not induce shivering. Shivering can increase heat production through the muscles by as much as 400%. That being said, if you’re dealing with a true heat emergency your cooling effects (coupled with vigorous massage) will likely not induce shivering very quickly. The only patient’s that I’ve cooled, and induced shivering on have been febrile infants.

This will probably seem obvious, but if you know you’ll be riding in the heat of the day, make sure you start the ride hydrated.

If you start to notice performance declines, especially regarding decision making and reaction time, focus on hydrating. If you start to notice cramping and loss of muscle acuity, focus on replacing electrolytes. Ideally, do both at the same time.

If you’re thirsty, you’re already about a liter low.

I talked a decent amount about losing sweat (hyponatremia), but it’s important to note that true hyponatremia doesn’t occur very often with short durations of exercise (< 2 hours), and is usually only present with multiple days of high sweating or ultra-endurance events. But, it does happen (usually to the lesser conditioned athletes).

Some heatstroke victims will develop euphoria, and will want to continue exercising.

When trying to rehydrate a heatstroke patient, try to avoid solutions that have a high concentration of carbohydrates, which may delay fluid absorption